First of all you are right in identifying the gospel for our justification unto eternal life in 1 Corinthians 15:1-4. This is all that has to be believed to be saved.
Amen, just as Jesus Christ Himself had preached (Luke 24:46-47).
Christ has accomplished a complete salvation not an initial salvation.
Note that the ideas of initial salvation and ultimate salvation do not have to be explicitly referred to in the Bible as "initial salvation" and "ultimate salvation" for them to be true and supported by the Bible, just as, for example, the ideas of the unity of God and the Trinity do not have to be explicitly referred to in the Bible as "the unity" and "the Trinity" for them to be true and supported by the Bible (John 10:30, John 1:1,14, Isaiah 45:5, Matthew 28:19, Acts 5:3-4).
For in the Bible, the difference between initial salvation and ultimate salvation is that initial salvation is by grace through faith in Jesus Christ without any works at all on our part (Romans 4:1-5, Ephesians 2:8-9, Titus 3:5; 2 Timothy 1:9), while other passages show that Christians must have both faith and continued works of faith (1 Thessalonians 1:3, Galatians 5:6b, Titus 3:8) (not works of the letter of the Old Covenant Mosaic law), if they are to obtain ultimate salvation (Romans 2:6-8, James 2:24, Matthew 7:21, Matthew 25:26,30, Philippians 2:12b, Philippians 3:11-14; 2 Corinthians 5:9, Hebrews 5:9, Hebrews 6:10-12; 2 Peter 1:10-11, John 15:2a; 1 John 2:17b). For Christians must continue to do righteous deeds if they are to continue to be righteous (1 John 3:7, James 2:24,26). And there is no assurance that Christians will choose to do that, instead of wrongly employing their free will to become utterly lazy without repentance, to the ultimate loss of their salvation (Matthew 25:26,30, John 15:2a).
Also, in the Bible, the difference between initial salvation and ultimate salvation is that initial salvation is the salvation which Christians have now (Ephesians 2:5), in their mortal bodies, while ultimate salvation is the salvation ready to be revealed in the last time (1 Peter 1:5), and which is always drawing nearer (Romans 13:11), that salvation which Christians are still hoping for (1 Thessalonians 5:8, Romans 8:23-25, Mark 10:30), and which Jesus Christ will bring to obedient Christians at His future, Second Coming (Hebrews 9:28, Hebrews 5:9), when He will resurrect (if dead) or change (if alive) their mortal physical bodies into immortal physical bodies just like the immortal physical body which Jesus obtained at His resurrection on the third day after His death (Luke 24:39,46; 1 Corinthians 15:3-4,21-23,51-53, Philippians 3:21, Romans 8:23-25, Philippians 3:11-14).
Hebrews 3:6,12,14 is addressed to Jews who are under the law and covenant and must endure to the end to be saved to enter the Davidic kingdom on earth.
Note that the book of Hebrews is about the Church (Hebrews 2:12, Hebrews 12:23), the body of Jesus Christ (Hebrews 13:3, cf. 1 Corinthians 12:25-27).
That's why the book of Hebrews says that the BOC is not under the Mosaic law and covenant (Hebrews 7:18-19, Hebrews 8:6-7).
Colossians 1:23 is saying that we need to continue in the faith to be holy and unblameable and unreproveable in his sight. This is not saying we lose salvation this is talking about our conduct being holy before God and being conformed to the image of Christ.
Note that the one is required for the other, ultimately (Romans 2:6-8).
Romans 2:6-8 is speaking of the judgment of God....this is how he will judge those who have not placed faith in Christ.
Note that it is also how He will judge real Christians (2 Corinthians 5:10-11).
1 Thessalonians 1:3 does not even say if you don't do all this you are going to hell....Paul is giving thanks for their labours in God's sight.
But note that Christian labours are required in order to be ultimately accepted by Jesus (2 Corinthians 5:9).
Hebrews 10:26-29 is addressed to Jews under the covenant and undergoing persecution and considering reverting back to the Levitical sacrifices. . .
Note again that the book of Hebrews is addressed to the Church (Hebrews 2:12, Hebrews 12:23), the body of Jesus Christ (Hebrews 13:3, cf. 1 Corinthians 12:25-27).
Also, regarding Hebrews 10:26-29, see post #174 above.
Mark 16:16 The twelve were told to go into all the world and preach the gospel of the kingdom and to be saved to enter into the Davidic kingdom on earth. This is not the commission of the church today it was to the twelve.
Note that it is (Acts 14:22)